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Smithsonian's National Zoo's Baby Gorilla: Spring 2011

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Human Origins Program at the Smithsonian's Museum of Natural History.

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Gorilla

Order: Primates
Infraorder: Catarrhini
Family: Hominidae

Species: Gorilla gorilla

Subspecies:
G. g. gorilla (western lowland)
G. g. diehli (Cross River)

Species: Gorilla beringei

Subspecies:
G. b. beringei
(mountain)
G. b. graueri (eastern lowland)

Some primatologists list one additional subspecies of mountain gorilla, and are proposing to separate the Bwindi population into a fifth gorilla subspecies.

Shy vegetarians, the world's largest primates face an uncertain future in Africa's remaining equatorial forests.

Physical Description

Gorilla of different subspecies vary in coat length, hair color, and jaw and teeth size. Individuals vary, but many western lowland gorillas (G. g. gorilla)—the subspecies to which the Zoo's gorillas belong—have brownish-gray coats, unlike the often blackish coats of the mountain (G. b. beringei) and eastern lowland (G. b. graueri) gorillas.

Generally, the mountain gorilla has longer hair than the other subspecies.

Western lowland gorillas have a more pronounced brow ridge, and ears that appear small in relation to their heads. They also have a different shaped nose and lip. Adult male gorillas’ heads look conical due to the large bony crests on the top (sagittal) and back (nuchal) of the skull. These crests anchor the massive muscles used to support and operate their large jaws and teeth. Adult female gorillas also have these crests, but they are much less pronounced. In comparison to the mountain gorilla, the western lowland gorilla has a wider and larger skull and the big toe of the western lowland gorilla is spread apart more from the alignment of his other four toes.

Like all great apes, gorillas’ arms are longer than their legs. When they move quadrupedally, they knuckle-walk, supporting their weight on the third and fourth digits of their curled hands. Like other primates each individual has distinctive fingerprints.

Lowland gorilla hair is short, soft, and very fine. There is no under fur (a thick layer of insulating hair close to the skin, such as on dogs or minks). Lowland gorillas’ coats are suited for warm, moist forest habitats. Mountain gorillas are more shaggy and thick-furred due to the colder temperatures at high altitudes.

Size

The eastern lowland gorilla is the largest. Adult male gorillas have silvery white "saddles" that inspired the name "silverback" for these animals.

On two legs, adult male gorillas stand about five a half feet tall (rarely a bit taller). They weigh between 300 and 400 pounds. Females are smaller, standing up to five feet tall and averaging about 200 pounds. Zoo animals are often heavier.

Geographic Distribution

Western lowland gorillas live in lowland tropical forests in Cameroon, the Central African Republic, the Republic of Congo, the Democratic Republic of Congo, Equatorial Guinea, Gabon, Angola, and Nigeria.

Eastern lowland gorillas, also called Grauer's gorillas, live in tropical forests from low elevations up to 8,000 feet in the Democratic Republic of the Congo (formerly Zaire) and along the border with Uganda and Rwanda.

Mountain gorillas, the rarest of the subspecies, hang on in mountain forests (up to 11,000 feet) at the borders of Rwanda, Uganda, and the Democratic Republic of the Congo.

Status

Western lowland and Cross River gorillas are listed as critically endangered on the IUCN Red List of Threatened Species. Eastern lowland and mountain gorillas are listed as endangered on the Red List.

Habitat

Gorillas live in moist tropical forests, often in secondary, or re-growing, forests or along forest edges, where clearings provide an abundance of low, edible vegetation. Mountain gorillas range up into cloud forest.

Diet in the Wild

Gorillas are primarily herbivorous, eating the leaves and stems of herbs, shrubs, and vines. In some areas, they raid farms, eating and trampling crops. They also will eat rotten wood and small animals.

The diet of western lowland gorillas also includes the fleshy fruits of close to a hundred seasonally fruiting tree species; the diets of other gorilla subspecies include proportionally less fruit. Gorillas get some protein from invertebrates found on leaves and fruits. Adult male gorillas eat about 45 pounds (20 kg) of food per day. Females eat about two-thirds of that amount.

Reproduction

Female gorillas reach maturity at seven or eight years old, but they usually don't breed until ten years or older.

Due to competition between males for access to females, few wild males breed before they reach 15 years old. Eight and a half months after mating, a female gives birth to one young, which can usually walk within three to six months. Young are usually weaned by three years old, and females can give birth every four years.

Upon reaching sexual maturity, between ages seven and ten, young gorillas strike out on their own, seeking new groups or mates. Zoo gorillas may reach sexual maturity before seven years old, and may have young every two to three years.

Life Span

Gorillas may live about 35 years in the wild, and up to 54 in zoos.

Behavior

Gorillas live in groups. Each group usually contains one or more silverbacks and two to ten females and young. Newly established silverbacks may kill young not sired by them, but otherwise, gorilla family life is mostly peaceful. Bloody battles sometimes occur between silverbacks when they square off to compete over female groups or home ranges. Gorillas spend their mornings and evenings feeding, usually covering only a small area of forest at a time. Groups spend the middle of the day sleeping, playing, or grooming (females groom their young or a silverback). At night, gorillas fashion nests of leaves and branches on which to sleep; unweaned infants sleep in their mothers' nests.

Social Structure

Gorillas are behaviorally flexible. This means that their behavior and social structure is not set in stone; there is great variety. The information below should only be used as a general guide.

Gorillas live in groups, or troops, from two to more than 30 members. Western lowland data seem to indicate smaller group sizes, averaging about five individuals. Groups are generally composed of a silverback male, one or more black back males, several adult females, and their infant and juvenile offspring. This group composition varies greatly due to births and deaths and to the immigration and emigration of individuals.

Mature offspring typically leave their natal group to find a mate. At about eight years old, females generally emigrate into a new group of her choosing. She seems to choose which silverback to join based on such attributes as size and quality of his home range, etc. This seems to be related to the silverback’s size, but not always. A female may change family groups a number of times throughout her life. When leaving their natal group, some sexually mature males may attempt to replace the silverback in an already established group. However, they usually spend a few years as solitary males. Nevertheless, a new troop can be easily formed when one or more non-related females join a lone male.

The group is led by the adult, dominant, silverback male. He has exclusive breeding rights to the females. At times he may allow other sub-adult males in the group to mate with females. The silverback mediates disputes and also determines the group’s home range. He regulates what time they wake up, eat and go to sleep.

Gorillas are most active in the morning and late afternoon. They wake up just after sunrise to search for food, and then eat for several hours. Midday, adults take a siesta and usually nap in a day nest while the young wrestle and play games. After their midday nap they forage again. Before dusk each gorilla makes its own nest, infants nest with their mothers.

All gorillas over three years make nests, day nests for resting and night nests for sleeping. Infants share their mothers’ nests. Gorillas form nests by sitting in one place and pulling down and tucking branches, leaves, or other vegetation around themselves. Adult males usually nest on the ground. Females may nest on the ground or in trees. Juveniles are more apt to nest in trees. Studies of western lowland gorillas have shown that the number of nests found at a site does not necessarily coincide with the number of weaned animals observed in a group.

The western lowland gorilla is characterized as a quiet, peaceful, and non-aggressive animal. They never attack unless provoked. However, males do fight over acquisition and defense of females, and the new leader of a group may kill unrelated infants. This causes the females to begin cycling sooner. An adult male protecting his group may attempt to intimidate his aggressor by standing on his legs and slapping its chest with cupped or flat hands while roaring and screaming. If this elaborate display is unsuccessful and the intruder persists, the male may rear his head back violently several times. He may also drop on all fours and charge toward the intruder. In general, when they charge they do not hit the intruder. Instead, they merely pass them by. This demonstration of aggression maintains order among separate troops and reduces the possibility of injury. It is thought that size plays an important role in determining the winner of an encounter between males (the larger male wins). Because of gorilla variability, some or all of these behaviors may not be seen.

Gorillas exhibit complex and dynamic relationships. They interact using grooming behaviors, although less than most other primates. Also affiliation may be shown by physical proximity.

Young gorillas play often and are more arboreal than the large adults. Adults, even the silverback, tolerate infant play behavior. He also tolerates, to a lesser extent, and often participates in the play of older juveniles and black back males.

The duration and frequency of sexual activity in gorillas are low in comparison to other great apes. The silverback has exclusive mating rights with the adult females in his group. The reproductive success of males depends upon the maintenance of exclusive rights to adult females. The female chooses to mate with the silverback by emigrating into his family group. Normally quiet animals, some gorillas are unusually loud during copulation.

Communication

Gorillas communicate using auditory signals (vocalizations), visual signals (gestures, body postures, facial expressions), and olfactory signals (odors). They are generally quiet animals, grunting and belching, but they may also scream, bark, and roar. Dian Fossey heard 17 different kinds of sounds from mountain gorillas. Other scientists have heard 22 different vocalizations, each seeming to have its own meaning. Gorillas crouch low and approach from the side when they are being submissive. They walk directly when confident and stand, chest beat (actually they slap with open hands), and advance when being aggressive.

Past/Present/Future

Until several decades ago, gorilla populations enjoyed the seclusion of vast tracts of forest. Today, Africa's growing population puts many pressures on these declining primates. Logging roads snake into forests, opening frontiers to settlers and loggers, while hunters kill or capture gorillas for their meat, parts (sometimes sold as souvenirs), or because the animals raid farm fields. Gorilla meat is eaten by hunters and loggers, and is also sold in city markets and restaurants.

While protection laws exist in most countries still inhabited by gorillas, enforcement is often lacking. Civil wars in Rwanda and the Democratic Republic of Congo have harmed conservation efforts in these countries and opened parks to poachers. Gorillas also stumble into snares set for other animals, and may be killed or injured. Increased political stability, better public awareness, and carefully protected parks would go a long way toward reversing the gorillas' decline.

Outbreaks of the Ebola virus and increased hunting led the IUCN to move the western lowland gorilla from endangered to critically endangered status in 2007. In August 2008, the Wildlife Conservation Society released a census showing that more than 125,000 western lowland gorillas are living in two adjacent areas of the northern part of the Republic of Congo. Previously, it was thought there could be fewer than 50,000 of these gorillas.

With a population of fewer than 300 individuals, Cross River gorillas are listed as critically endangered.

A Few Gorilla Neighbors

Leopard (Panthera pardus): The only animal in its range, aside from humans, that can harm an adult gorilla, although these animals rarely tangle with each other.

African elephant (Loxodonta africana): By downing trees, forest-dwelling elephants help create gorilla feeding areas.

African gray parrot (Psittacus erithacus): Despite a large range, this forest parrot is disappearing from many areas due to capture for the cage bird trade and forest cutting. By saving gorilla habitat, we protect these and many other animals.

Fun Facts

Despite their size and current popularity, gorillas remained a mystery to people living outside of Africa until a missionary described them in 1847. After chimpanzees, gorillas are our closest relatives, sharing about 98 percent of our genes.